It is frequently necessary in industry to position an object with a high degree of accuracy relative to a plane. This can be necessary during the machining of a single piece or during the assembly of a plurality of pieces. Such a positioning carried out at two different points likewise makes it possible to measure the length separating these two points.
The methods of positioning and measuring objects by optical means are conventional. In most cases these methods comprise locating points on the object concerned by means of optical peaks. Measurements of length are obtained either relative to fixed marks or relative to mobile marks.
More recently, for example in U.S. Pat. No. 4,332,475, it has been proposed that the dimensions of an object should be measured by sweeping it with a light-beam and measuring the time taken by the beam to pass from one of the edges of the object to the other.
This last-named method of measurement is limited to objects whose dimensions are inferior to the amplitude of the sweep of the beam, and further limited in that the detection of the objects comprises an area of uncertainty due to the width of the beam performing the said sweep.